Canon SD970 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Nikon L820 has a
16.0MP 1/2.3-inch (6.16 x 4.62 mm ) sized BSI-CMOS sensor .
Below you can see the SD970 IS and L820 sensor size comparison.
Canon SD970 IS and Nikon L820 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.